Dollar bill is good, but I don't always have any money on me (seldom, in fact). I have an old tyvek express mail envelope cut down a bit and in my repair kit. It takes very little room when folded up and is probably even better than a dollar bill for making a boot.

Ditto other opinions though; you could have been riding with kevlar belted tires and tire liners and puncture resistant tubes and that flat still would have happened.